The alert issued by the California Independent System Operator is in effect from Friday through Sunday. During that time, consumers are asked to set air conditioning at 78 degrees or higher or turn it off while away from home, turn off unnecessary lights and appliances and restrict use of major appliances to the morning and late evening.

Flex alerts are a measure to encourage energy conservation at times when the statewide energy grid could become strained, leading to outages.

The National Weather Service has issued an excessive heat warning through Friday for several spots in Los Angeles, Ventura and Santa Barbara counties, including mountain ranges and valleys, with triple-digit heat projected in much of the region.

The heat wave comes as Southern California has been coping without the 2,200 megawatts of power normally generated by the San Onofre nuclear plant. The plant has been out of service for more than six months due to equipment problems.

On top of that, ISO spokeswoman Stephanie McCorkle said another large power plant in Southern California suffered an equipment failure that took a generating unit out of service Wednesday night. McCorkle declined to say which plant the failure occurred at because the information is considered "market sensitive."

Officials have implemented contingency plans to replace the lost power from San Onofre, including firing up mothballed generating units at another plant and completing the Sunrise Powerlink and Barre-Ellis transmission lines to allow for importing more power into the region.

At the beginning of the summer, officials had said that they expected Southern California could get through the hot months without outages unless the region is hit with an unusual heat event.

The projected statewide energy demand for Thursday was 47,125 megawatts, close but just short of such an event, McCorkle said.